The present invention relates to a method for preparing cyclotetrasiloxanes from chlorosilanes without the concomitant formation of hydrogen chloride or hydrochloric acid. More specifically the present invention relates to a method for preparing cyclotetrasiloxanes, particularly mixed cyclotetrasiloxanes, by heating a mixture of a cyclotrisiloxane, a chlorosilane and an acyloxy compound and coproducing an aliphatic chloride and/or an acyl chloride.
Cyclotetrasiloxanes, particularly cyclotetradiorganosiloxanes such as octamethylcyclotetrasiloxane, are valuable materials because they can be further polymerized to prepare higher molecular weight silicone compositions, such as oils, gums and resins, and because they are useful, without further processing, as fluids.
Mixed cyclotetrasiloxanes, particularly mixed cyclotetradiorganosiloxanes such as heptamethylvinylcyclotetrasiloxane, are also valuable materials for the reasons give above and, additionally, because they can be polymerized to prepare higher molecular weight silicone copolymers having a specific composition and structure.
It is well known that dihydrocarbyldichlorosilanes can be hydrolyzed to prepare poly(dihydrocarbylsiloxanes) and hydrochloric acid. This method, although finding extensive commercial use, has several disadvantages. For example, the by-produced hydrochloric acid is an undesired by-product because it is corrosive, it cannot be used directly to form more dihydrocarbyldichlorosilane and it frequently reacts further with the hydrocarbyl radicals, such as methyl and vinyl radicals. Furthermore, this hydrolysis process does not produce only cyclopolysiloxanes, much less only cyclotetrasiloxanes, but rather a mixture of various linear and cyclic polysiloxanes. Also, this hydrolysis process cannot be used to prepare cyclopolysiloxanes bearing water-sensitive radicals such as silicon-bonded chlorine atoms or acyl chloride radicals. Additionally, this hydrolysis process cannot be used to prepare mixed cyclotetrasiloxanes having a specific composition, because the random joining of hydrolyzed silane molecules forms random siloxanes and, hence, random cyclotetrasiloxanes.
Speier, in an application for U.S. patent, titled "Preparation of Cyclotetrasiloxanes and Aliphatic Chlorides and/or Acyl Chlorides", Ser. No. 282,289, assigned to the assignee of this invention and filed on July 10, 1981, claims a method for preparing cyclotetrasiloxanes which has many advantages over the processes of the art and avoids many of the disadvantages of the processes of the art. The method of Speier does not coproduce hydrogen chloride or hydrochloric acid; but, rather, an aliphatic chloride and/or an acyl chloride. In addition to cyclotetrasiloxanes having water-insensitive radicals, the method of Speier also provides cyclotetrasiloxanes which bear silicon-bonded radicals which are water-sensitive, such as chlorine atoms and acyl chloride radicals. However, the method of Speier cannot provide mixed cyclotetrasiloxanes having a specific composition.
Other methods for preparing cyclotetrasiloxanes have been disclosed in the art. Generally these methods comprise the condensation reaction of a silane reactant with a linear trisiloxane reactant and are exemplified by the disclosure of U.S. Pat. No 3,317,578; 3,328,345; 3,347,895 and 3,358,009. Typically, one of the reactants contains silicon-bonded chlorine atoms and the other reactant contains silicon-bonded hydroxyl radicals, the inter-reaction of which produces hydrogen chloride and the desired cyclotetrasiloxane. As noted above hydrogen chloride is an undesirable by-product; it is in these processes as well. Additionally, the cited patents teach that the by-produced hydrogen chloride leads to the formation of linear siloxanes at the expense of the desired cyclotetrasiloxane unless it is immediately removed from the reaction mixture; a requirement that is difficult to meet or that produces additional disadvantages. For example, rapid removal of hydrogen chloride can be achieved by using an amine in the reaction mixture as an HCl scavenger; however, the resulting amine hydrochloride salt is difficult to remove. On the other hand, HCl can be removed by aeration or boiling; however, this is a relatively slow method of removing HCl and undesirable acid catalyzed reactions of the cyclotetrasiloxanes often occur. Additionally, this condensation reaction method for producing cyclotetrasiloxane has the further disadvantage of being unsuitable for preparing cyclotetrasiloxanes which bear silicon-bonded acyl chloride radicals because they react with silicon-bonded hydroxyl radicals.